The invention relates to an ion implantation apparatus, and particularly to a technique for suppressing a charge-up in a product wafer during an ion implantation, and also to a method of producing a semiconductor device by using the ion implantation apparatus.
Conventionally, some ion implantation apparatus have a wafer processing chamber (not shown). As shown in FIG. 5 in a simplified manner, such a wafer processing chamber executes ion implantation on plural product wafers 2 while integrally revolving the product wafers at a high speed of, for example, about 2,000 rpm in a locus passing through an irradiation range of ion beams 1. A wafer wheel 5 having plural wafer holders 4 is disposed in the wafer processing chamber. The wafer holders radially elongate from a rotating shaft 3 and respectively hold the product wafers 2. The rotating shaft 3 is grounded and subjected to a scanning operation while rotating the rotating shaft 3, whereby the whole faces of the product wafers 2 are irradiated with the ion beams 1.
Such an ion implantation apparatus of the prior art has the following disadvantage. Although illustration is omitted, when the surface of each of the product wafers 2 to be subjected to ion implantation is covered by an insulating film such as a silicon oxide film, a charge-up occurs during ion implantation. Specifically, positive charges are accumulated in the vicinity of the surface of the insulating film, and, when the accumulated amount of the positive charges exceeds the withstand voltage of the insulating film, a dielectric breakdown occurs. In the prior art, in order to avoid the disadvantage, a countermeasure (not shown) is taken as disclosed in Unexamined Japanese patent publication (Kokai) 63-207126. Namely, after a shield metal film made of aluminum or the like is formed on the surface of each of the product wafers 2, ion implantation is executed and the metal film is then thoroughly removed away.
When the prior art technique is employed, however, a shield metal film must be specially formed and then removed away. The steps for the formation and removal require extra labor and costs. Furthermore, the insulating film may be damaged in the step of removing the metal film, and there is a possibility that aluminum forming the metal film enters the product wafer 2 made of silicon or a junction portion. In such a case, an energy level due to metal contamination is formed, so that device properties are impaired.
After ion implantation, the product wafers 2 may be irradiated with an electromagnetic wave charged in a polarity which is opposite to that of charges accumulated in the surfaces of the product wafers 2. In the example described above, positive charges are accumulated in the surfaces of the product wafers 2. When the product wafers are irradiated with electrons of the opposite polarity so as to be electrically neutralized, a charge-up can be prevented from occurring.
In the charge neutralizing method, however, further production steps and apparatuses are required. Moreover, the irradiation amount of electrons must be set in accordance with the amount of charges accumulated in the surfaces of the product wafers 2. Therefore, it is difficult to optimize the method, and the neutralization cannot be always adequately conducted.
The above-mentioned charge accumulation in the surface of a wafer product is noticeable in a case where ions are implanted at a high concentration to a substrate, such as the formation of source and drain regions of a semiconductor device, particularly, a MOS semiconductor device of the extension structure. It has been requested to develop an effective countermeasure.